This invention relates to the field of voltage comparators and, more particularly, to a low-power voltage comparator that may be used in analog-to-digital converters (ADCs).
A voltage comparator is a generic electronic device that compares the voltages of its two analog inputs at a specified time and provides a digital output based on the comparison result. A comparator is a key component in ADCs which are used commonly in communication, radar, medical, and instrumentation systems. Major performance criteria of a comparator are power, sampling rate, sensitivity, input bandwidth, and voltage range. High-speed and low-power comparators are always in high demand, especially in mobile systems. They are conventionally implemented with the families of bipolar junction transistors (BJTs) or field effect transistors (FETs). A typical comparator may include seven transistors and 3 resistors, and consumes about 10 mW to 100 mW of power depending on the sampling rates.
For many applications, the battery power provided to the comparator is finite. Therefore, there is always a need for comparators that include transistors that can operate using less power.
Further, each transistor contributes its parasitic capacitance and resistance to the overall comparator circuit. Therefore, as the number of transistors increases, the sampling rate of the analog-to-digital converter using the comparator decreases. Accordingly, there is a need for comparator circuits that use fewer transistors and therefore contribute less parasitics. There is also a need for faster transistors that can increase the sampling rate of the comparator circuits.
Innovative approaches to increase the speed of ADCs while lowering their power consumption include using a superconductor, a single electron transistor (SET), or a quantum tunneling transistor based device, such as a resonant tunneling diode (RTD).
Quantum tunneling transistors may be used as on-off switches. The quantum tunneling transistors exploit an electron's ability to pass through normally impenetrable barriers, allowing for fabrication of faster transistors that can be mass-produced with current nanotechnology. The flow of electrons is controlled between two GaAs layers separated by an AlGaAs barrier. Although the electrons in GaAs ordinarily do not have enough energy to enter the AlGaAs barrier, the layers are very thin so that they are comparable in size to the electron wavelength. At small thicknesses, the electrons are considered as waves rather than particles and can spread into the barrier and, with an appropriate voltage applied, proceed out the other side. In this process, the electron waves do not collide with impurity atoms. This is in contrast to a traditional transistor's particle-like electrons, which are slowed down by collisions with the impurity atoms doping the lattice. Transistors using this approach, switch on and off many times faster than current GaAs channel field effect transistors.
RTD devices take advantage of quantum mechanical effects such as electron resonant tunneling. An example of a RTD is disclosed in U.S. Pat. No. 5,825,049, to Simmons et al. A regular RTD is a two terminal device which acts similarly to a diode and has a resistance that varies nonlinearly with an applied bias. The lack of a third terminal that imposed a limitation on the usefulness of a RTD was addressed by the '049 patent that discloses a device with a gate. However, fabrication of such devices is not trivial. As complexity of fabrication increases, yield decreases.
Therefore, a need still exists for a low power voltage comparator using components with a simplified fabrication process that may be used in ADC devices.